Recently, an increasing amount of material which is prepared by laminating a thermoplastic resin film such as polyester resin film to both surfaces of a metal sheet has been used for heavily formed cans such as drawn cans and drawn and stretch-formed cans. A resin coated metal sheet used for such heavily formed cans is manufactured by a method of heat bonding a resin film to a metal sheet or a method of laminating a resin film into a metal sheet by means of a thermosetting adhesive therebetween. There arises no particular problem in both methods as far as the resin film is laminated on the metal sheet at low speed. However, when the resin film is laminated onto the metal sheet at a high speed of at least 200 m/min., the resultant resin coated metal sheet has bubbles included between the laminated resin film and the metal sheet and a ratio of the thus included bubbles to the resin coated metal sheet comes to about 10 to 30% by an area ratio. Such inclusion of bubbles not only causes decrease in the adhesion after forming of the laminated resin film but also causes surface roughness of the resin coated metal sheet, which results in flaws of the resin coated metal sheet at the time of forming. Therefore, the lamination speed at the time of laminating a resin film to a metal sheet has been limited to a low speed form the view of reducing bubbles at present.
In the method of continuously laminating a resin film onto a metal sheet, various techniques for preventing bubbles from being included between the laminated resin film and the metal sheet have been proposed as follows.
(1) Japanese Patent Laid-open Publication No. HEI-5-269857 discloses a method in which the surface roughness of a metal sheet is set to 0.5 .mu.m or less, and a space defined by an intermediate roll and a seal roll which are disposed before a pressure roll is conditioned under a reduced pressure of 50 Torr or less.
(2) Japanese Patent Laid-open Publications Nos. HEI-6-8335 and HEI-6-8336 disclose a method in which the diameter of a laminating roll, the thickness of a rubber lining of the laminating roll, and the hardness of the rubber roll are respectively fixed, and a backup roll having a larger diameter than the laminating roll is further provided.
(3) Japanese Patent Laid-open Publication No. HEI-6-270363 discloses a method in which a first pressure roll is a steel roll by which a laminate material is roller with a reduction ratio of 0.5 to 2.0% and a second pressure roll is a rubber lining roll, and the laminate material is press-bonded with a nip width of 10 m or more.
(4) Japanese Patent Laid-open Publication No. HEI-7-186353 discloses a method in which a seal roll for nipping a metal sheet strip is disposed on an upstream side of a pressure roll, and intermediate roll is further provided in contact with these rolls, and a space defined by these rolls is conditioned under a reduced pressure, and a resin film is laminated on the metal sheet strip under such conditions.
(5) Japanese Patent Laid-open Publication No. HEI-7-214724 discloses a method in which a gas is blown from a gas jet nozzle against a surface of a metal sheet, which is advancing, in the opposite direction to the advance direction of the metal sheet directly before a resin film is laminated onto the metal sheet.
Considering the case of laminating a resin film to a metal sheet continuously at high speed, these methods (1) to (5) are not sufficiently suitable as the method of manufacturing a resin coated metal sheet for cans which requires excellent formability.
Therefore, it is the object of the present invention to solve the above mentioned problems in the manufacture of a resin coated metal sheet for cans. That is, the object of the present invention is to provide a method and an apparatus for manufacturing a resin coated metal sheet, which make it possible to reduce bubbles included between a laminated resin film and a metal sheet when the resin film is continuously laminated onto the metal sheet at a high speed of at least 200 m/min. and which make it also possible to impart excellent adhesion after forming to the laminated resin film of the resin coated metal sheet thus manufactured.